Fixed plotting error when there's no config

predtuner/modeledapp.py

@@ -460,6 +460,8 @@ class ApproxModeledTuner(ApproxTuner):
             def qos_speedup(conf):
                 return conf.validated_qos if validated else conf.qos, conf.speedup
 
+            if not confs:
+                return np.zeros((2, 0))
             sorted_points = np.array(
                 sorted([qos_speedup(c) for c in confs], key=lambda p: p[0])
             ).T